To be a conventionalist about a specific obligation or right is to believe that the obligation or right is dependent on the existence of a social practice. A conventionalist about property, for example, believes that a moral right to property is generated by conventional norms rather than by any natural right. One problem with dominant conventionalist theories is that they do not adequately justify conventional moral claims. They can justify why it is wrong to steal, for example, but they do not justify the claim that you have on me to not steal from you. As a remedy, this article develops and defends the Principle of Legitimate Expectations. Suggested by John Rawls, this principle grants individuals a moral claim to what the rules of morally justified practices entitle them. This article addresses common objections to the principle to show how it can ground a wide range of conventional moral claims.